Type not assignable for string literal in signature

[WangHansen]

TypeScript Version: 3.8.3

Search Terms:
string literal
string alias
is not assignable to parameter of type

Expected behavior:
No type check errors

Actual behavior:

No overload matches this call.
  The last overload gave the following error.
    Argument of type 'tAlgorithm' is not assignable to parameter of type '"oct"'.
      Type '"EC"' is not assignable to type '"oct"'.

Related Issues:

Code

interface KeyStore {
    generate(kty: 'EC', crv?: string, parameters?: object, isprivate?: boolean): void;
    generate(kty: 'OKP', crv?: string, parameters?: object, isprivate?: boolean): void;
    generate(kty: 'RSA', bitlength?: number, parameters?: object, isprivate?: boolean): void;
    generate(kty: 'oct', bitlength?: number, parameters?: object): void;
}

type tAlgorithm = "RSA" | "EC" | "OKP" | "oct";

function generateKey(alg: tAlgorithm, crvOrSize: string | number) {
    return ({} as KeyStore).generate(alg, crvOrSize);
}

Output

"use strict";
function generateKey(alg) {
    return {}.generate(alg, "P-256");
}

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[WangHansen]

Another reproduction link: https://codesandbox.io/s/laughing-shockley-hlpr1?file=/src/index.ts

[DanielRosenwasser]

This clearly shouldn't work given that generateKey("EC", 128) can be substituted with

({} as KeyStore).generate("EC", 128)

and shouldn't match any overload.

[WangHansen]

@DanielRosenwasser Correct, but that is not what the error message is suggesting.
Also, what about the following?

interface KeyStore {
    generate(kty: 'EC', crv?: string, parameters?: object, isprivate?: boolean): void;
    generate(kty: 'OKP', crv?: string, parameters?: object, isprivate?: boolean): void;
    generate(kty: 'RSA', bitlength?: number, parameters?: object, isprivate?: boolean): void;
    generate(kty: 'oct', bitlength?: number, parameters?: object): void;
}

type tAlgorithm = "RSA" | "EC" | "OKP" | "oct";

function generateKey(alg: tAlgorithm) {
  return ({} as KeyStore).generate(alg);
}

This still gives the error.
Playground

Also, could you provide an alternative approach for this function if I want to pass string literals as a parameter of the function?

[DanielRosenwasser]

Sorry to not also link to the more general problem earlier on (it would've spared you some time): #1805 has some explanation on why the pattern is currently not supported.

Though, since then, I can sort of imagine us doing better given some of the work that went into #30779.

[DanielRosenwasser]

However, I think that even with the ideas of #30799 incorporated, it wouldn't help you write the original code easily without a type assertion or duplicate overloads.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.